The present invention relates to a radiation detecting apparatus using a scintillator which emits light on receiving radiation such as X-rays and .gamma.-rays, and, more particularly, to a radiation detecting apparatus suitable for use in an X-ray computed tomograph (hereinafter referred to as an "X-ray CT").
In an inspecting apparatus using radiation such as the X-ray CT, a radiation detector is required to detect radiation with high sensitivity. Especially, a radiation detector for use in a medical field is required to have a high sensitivity, in order to ensure that the radiation dose used is as small as possible, thereby minimizing the radiation injury to a patient. In other words, the radiation detector is required to have high signal-to-noise ratio. Further, in an X-ray CT, X-rays having passed through a to-be-examined body and thereby subjected to intensity modulation are detected by a plurality of radiation detectors at the same time. In this case, even if the energy intensity of the X-rays is distributed over a wide range, it is required that the radiation detectors are substantially equal in characteristic for X-rays to each other. In short, it is necessary to use radiation detectors having the same characteristics.
As the radiation detector of this kind, a detector has hitherto been known which is formed by combining a scintillator for converting radiation energy into visible light or near ultraviolet radiation and near infrared radiation with a photoelectric conversion element for converting light from the scintillator into an electric signal.
In, for example, U.S. Pat. No. 4,317,037 a radiation detector is proposed which is the combination of a scintillator formed of particles of a scintillator material, and a silicon photodiode. The respective characteristics of the scintillator particles may be slightly different. However, when the scintillator particles are used as one scintillator after having been well mixed, variations in characteristic of the scintillator are reduced. Additionally, in this proposed construction light generated in the scintillator can readily escape to the outside by tilting the scintillator or forming the scintillator of one or more layers.
In, for example U.S. Pat. No. 4,492,869 a radiation detector using a powder scintillator is proposed wherein the detector is superior in signal-to-noise ratio to conventional detectors of this kind.
A radiation detecting apparatus for use in an inspecting apparatus using radiation such as the X-ray CT, is formed by arranging a multiplicity of such radiation detectors with a radiation shielding plate between adjacent radiation detectors. More particularly, a thin plate made of a material having a large absorption coefficient for radiation such as tungsten, tantalum, or molybdenum is interposed between adjacent radiation detectors.
In order to obtain a high-precision, high-resolution, clear sectional image, the width of the radiation detector is preferably in the order of 1 to 3 mm, and it is required to hold a correct positional relationship between the radiation detectors and an X-ray source in a geometrical sense and to make variations in mechanical dimensions of radiation detectors due to the characteristic thereof as small as possible.
In a radiation detecting apparatus having a structure wherein the photodetectors are disposed fixedly on each of a facing pair of substrates at a predetermined interval and a shielding plate provided with a scintillator inserted between a facing pair of photodetectors to form a radiation detector, the geometrical position accuracy of the radiation detector depends upon the accuracy, with which the photodetectors are arranged. Therefore, it is difficult to obtain satisfactory results, and a ring-shaped artifact may be generated in a tomogram. Incidentally, the ring-shaped artifact is generated in the case when the sensitivity of specified ones of radiation detectors for forming the radiation detecting apparatus is different from the sensitivity of the remaining radiation detectors by a predetermined value or more. Further, in order to take out respective outputs of the photodetectors, it is necessary to draw out lead wires from the photodetectors separately and to perform terminal treatment for the drawn-out lead wires. Thus, the amount of work is increased. Further, when an X-ray CT provided with such a radiation detecting apparatus is operated, electric noise is caused by the vibration of the lead wires. Accordingly, not only is the quality of a reconstructed picture image degraded, but also the reliability of the X-ray CT is reduced. Further, the productivity of the radiation detecting apparatus is lowered.